The objective of this research is to determine changes in levels and ultrastructural location of elements associated with biological mineralization by electron microprobe analysis. Cells and matrix in early stages of bone formation are being studied in 7-day chick embryo femurs. Avian shell gland epithelium in the process of forming an egg shell will be compared with the resting shell gland. Particular attention is given to subcellular organelles involved in calcification, such as matrix vesicles and mitochondria. Maximum preservation of mineral constituents is achieved by the use of frozen thin-sections.